1. Field of the Invention
The present invention relates to current-excitation light-emitting elements, and also relates to light-emitting devices and electronic devices having such light-emitting elements.
2. Description of the Related Art
In recent years, a light-emitting element utilizing electroluminescence has actively been researched and developed. The basic structure of the light-emitting element is such that a light-emissive compound is sandwiched between a pair of electrodes. By applying a voltage to such an element, light emission can be obtained from the light-emissive compound.
Such a light-emitting element which is a self-luminous type has an advantage in that it has higher visibility of pixels than liquid crystal displays, and there is no need to use a backlight. Thus, such a light-emitting element is considered to be suitable for a flat panel display element. Besides, such a light-emitting element has advantages in that it can be formed to be thin and lightweight, and has quite fast response speed.
Furthermore, since such a light-emitting element can be formed in a film form, planar light emission can be easily obtained by forming a large-area element. This cannot be easily achieved with a point light source typified by an incandescent lamp or an LED, or with a line light source typified by a fluorescent lamp. Therefore, the light-emitting element has a high utility value as a plane light source that can be applied to lighting or the like.
The light-emitting elements using electroluminescence can be roughly classified into light-emitting elements whose light-emissive compound is an organic compound and light-emitting elements whose light-emissive compound is an inorganic compound. The invention relates to the former light-emitting elements whose light-emissive compound is an organic compound. In this case, when a voltage is applied to the light-emitting element, electrons and holes are injected from a pair of electrodes into a layer containing a light-emissive organic compound, whereby a current flows thereto. Then, the carriers (electrons and holes) are recombined and the light-emissive organic compound comes to an excited state. When the organic compound returns from the excited state to the ground state, light emission is obtained.
Because of such a mechanism, the light-emitting element is called a current-excitation light-emitting element. As the types of the excited states obtained by an organic compound, there are a singlet excited state and a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
Such a light emitting element has many material-dependent problems for improvement of the element characteristics, and in order to overcome the problems, improvement in element structure, development of materials, and the like have been conducted. For example, in Reference 1 (Tetsuo TSUTSUI, and eight others, Japanese Journal of Applied Physics, Vol. 38, L1502-L1504 (1999)), the luminous efficiency of a light-emitting element using a phosphorescent material is improved by providing a hole blocking layer.
However, since the hole blocking layer disclosed in Reference 1 is not durable, the light-emitting element has a short lifetime. Therefore, improvement in lifetime of the light-emitting element is desired. In view of the foregoing, it is an object of the invention to provide a light-emitting element having a long lifetime. It is another object of the invention to provide a light-emitting device and an electronic device having a long lifetime.